As we've already seen The Flash and Arrow together, as well as Arrow and Legends of Tomorrow, futher down the road it's possible there could be one mega crossover between all four shows.

While speaking with EW.com, Grant Gustin mentions he is hoping it's a possibility.

But viewers will get to watch Barry and Kara address an age-old question. Like The Flash and Superman before them in the comics, the two have a footrace to figure out who is faster. “It’s one of my favorite moments in this episode,” says Kreisberg. “It’s close.” Hopefully close enough that they’ll need a rematch next season, maybe with Green Arrow and the Legends as an audience. “That’s in the cards at this point,” says Gustin. “The whole point of us doing this crossover is setting up the fact that maybe next year we can cross over all of them.”

Watch two teasers below for The Flash and Supergirl episode "Worlds Finest."

Episode synopsis:

Kara gains a new ally when the lightning-fast superhero The Flash (Grant Gustin) suddenly appears from an alternate universe and helps Kara battle Siobhan, aka Silver Banshee, and Livewire in exchange for her help in finding a way to return him home.